Many hardwired communications systems use plug and jack connectors to connect a communications cable to another communications cable or to computer equipment. By way of example, high speed communications systems routinely use such plug and jack connectors to connect computers, printers and other devices to local area networks and/or to external networks such as the Internet. FIG. 1 depicts a highly simplified example of such a hardwired high speed communications system that illustrates how RJ-45 plug and jack connectors may be used to interconnect a computer 11 to, for example, a network server 20.
As shown in FIG. 1, the computer 11 is connected by a cable 12 to a communications jack 15 that is mounted in a wall plate 19. The cable 12 is a patch cord that includes a communications plug 13, 14 at either end thereof. Typically, the cable 12 includes eight insulated conductors. As shown in FIG. 1, plug 14 is inserted into an opening or “plug aperture” 16 in the front side of the communications jack 15 so that the eight contacts of communications plug 14 mate with the respective eight contacts of the communications jack 15. The communications jack 15 includes a wire connection assembly 17 at the back end thereof that receives the eight conductors from a second cable 18 that are individually pressed into slots in the wire connection assembly 17 to establish mechanical and electrical connections between each conductor of the second cable 18 and a respective one of eight conductive paths through the communications jack 15. The other end of the second cable 18 is connected to a network server 20 which may be located, for example, in a telecommunications closet of a commercial office building. Communications plug 13 similarly is inserted into the plug aperture of a second communications jack (not pictured in FIG. 1) that is provided in the back of the computer 11. Thus, the patch cord 12, the cable 18 and the communications jack 15 provide eight electrical paths between the computer 11 and the network server 20. These electrical paths may be used to communicate electrical information signals between the computer 11 and the network server 20.
When a signal is transmitted over a conductor (e.g., an insulated copper wire) in a communications cable, electrical noise from external sources may be picked up by the conductor, degrading the quality of the signal. In order to counteract such noise sources, the information signals in the above-described communications systems are typically transmitted between devices over a pair of conductors (hereinafter a “differential pair” or simply a “pair”) rather than over a single conductor. The two conductors of each differential pair are twisted tightly together in the communications cables and patch cords so that the eight conductors are arranged as four twisted differential pairs of conductors. The signals transmitted on each conductor of a differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors of the pair. When the signal is transmitted over a twisted differential pair of conductors, each conductor in the differential pair often picks up approximately the same amount of noise from these external sources. Because the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, the subtraction process may mostly cancel out the noise signal, and hence the information signal is typically not disturbed.
Referring again to FIG. 1, it can be seen that a series of plugs, jacks and cable segments connect the computer 11 to the server 20. Each plug, jack and cable segment includes four differential pairs, and thus a total of four differential communications paths are provided between the computer 11 and the server 20 that may be used to carry two way communications therebetween (e.g., two of the differential pairs may be used to carry signals from the computer 11 to the server 20, while the other two may be used to carry signals from the server 20 to the computer 11). The cascaded plugs, jacks and cabling segments shown in FIG. 1 that provide connectivity between two end devices (e.g., computer 11 and server 20) is referred to herein as a “channel.” Thus, in most high speed communications systems, a “channel” includes four differential pairs. Unfortunately, the proximities of the conductors and contacting structures within each plug-jack connection (e.g., where plug 14 mates with jack 15) can produce capacitive and/or inductive couplings. These capacitive and inductive couplings in the connectors (and similar couplings that may arise in the cabling) give rise to another type of noise that is known as “crosstalk.”
In particular, “crosstalk” refers to unwanted signal energy that is capacitively and/or inductively coupled onto the conductors of a first “victim” differential pair from a signal that is transmitted over a second “disturbing” differential pair. The induced crosstalk may include both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both types of crosstalk comprise an undesirable noise signal that interferes with the information signal that is transmitted over the victim differential pair.
Crosstalk that arises between two differential pairs that are part of the same channel is typically referred to as “internal” crosstalk. Because communications cables are often bundled together for routing through the walls, floors and/or, ceilings of buildings and/or because communications connectors are often located in very close proximity to each other in, for example, patch panels and switches, crosstalk may also occur between one or more differential pairs of a first channel and one or more differential pairs of a second channel. Such crosstalk between differential pairs of different channels is typically referred to as “alien” crosstalk.
While methods are available that can significantly reduce the effects of crosstalk within communications cable segments, the communications connector configurations that were adopted years ago—and which still are in effect in order to maintain backwards compatibility—generally did not arrange the contact structures so as to minimize crosstalk between the differential pairs in the connector hardware. For example, pursuant to the ANSI/TIA-568-C.2 standard approved Aug. 11, 2009 by the Telecommunications Industry Association (also known as the Category 6a standard), in the connection region where the contacts of a modular plug mate with the contacts of the modular jack (referred to herein as the “plug-jack mating region”), the eight contacts 1-8 of the jack must be aligned in a row, with the eight contacts 1-8 arranged as four differential pairs specified as depicted in FIG. 2. As known to those of skill in the art, under the TIA/EIA 568 type B configuration, contacts 4 and 5 in FIG. 2 comprise pair 1, contacts 1 and 2 comprise pair 2, contacts 3 and 6 comprise pair 3, and contacts 7 and 8 comprise pair 4. As is apparent from FIG. 2, this arrangement of the eight contacts 1-8 will result in unequal coupling between the differential pairs, and hence both NEXT and FEXT is introduced in each connector in industry standardized communications systems.
As hardwired communications systems have moved to higher frequencies in order to support increased data rate communications, crosstalk in the plug and jack connectors has became a more significant problem. To address this problem, communications jacks now routinely include crosstalk compensation circuits that introduce compensating crosstalk that is used to cancel much of the “offending” crosstalk that is introduced in the plug-jack mating region as a result of the industry-standardized connector configurations. Typically, so-called “multi-stage” crosstalk compensation circuits are used. Such crosstalk circuits are described in U.S. Pat. No. 5,997,358 to Adriaenssens et al., the entire content of which is hereby incorporated herein by reference as if set forth fully herein.
Another important parameter in communications connectors is the return loss that is experienced along each differential pair (i.e., differential transmission line) through the connector. The return loss of a transmission line is a measure of how well the transmission line is impedance matched with a terminating device or with loads that are inserted along the transmission line. In particular, the return loss is a measure of the signal power that is lost due to signal reflections that may occur at discontinuities (impedance mismatches) in the transmission line. Like crosstalk, return loss is typically expressed as a ratio in decibels (dB) as follows:
      R    ⁢                  ⁢          L      ⁡              (        dB        )              =      10    ⁢                  ⁢          log      10        ⁢                  P        i                    P        r            where RL(dB) is the return loss in dB, Pi is the incident power and Pr is the reflected power. High return loss values indicate a good impedance match (i.e., little signal loss due to reflection), which results in lower insertion loss values, which is desirable.
Another important parameter in such communications connectors is the balance that is experienced along each differential transmission line through the connector. The balance of a transmission line is a measure of how well the individual conductors of the transmission line are matched with each other and with their surroundings. In particular, the balance is a measure of the differential signal power that is lost due to conversion into common mode signal power that may occur in sections of mismatch in the differential transmission line. By electromagnetic reciprocity, it is also a measure of how much susceptibility the differential transmission line has to external noise sources as described above.